Having seen John-Joe O'Toole dismissed in the first-half and then conceded just after the break to Vadiane Oliver, Town looked to be heading for their second away defeat of the week.
Top goalscorer Richards, however, kept his composure in stoppage time after Ivan Toney had been fouled in the box.
A point means Northampton are eight points off the play-offs in League Two and assistant manager Alan Knill spoke to the official club website after the game.
He said: "It was a fantastic effort by the ten men and with the support we had here today it felt like a home game. It was a great effort by the team and the fans."